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Part 2. Humans are (Blank) -ogamous: Promiscuity

This is the second part on the evolution of human mating behavior, comparing evidence for promiscuity and pair-bonding in our species. Please see the Introductionhere.

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“I’ll be frank. True monogamy is rare. So rare that it is one of the most deviant behaviors in biology.” (Olivia Judson 2002: 153)

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In their best-selling book, Sex at Dawn, Chris Ryan and Cacilda Jethá suggest that there is a good amount of direct and circumstantial evidence that extended monogamy does not come easily for humans, and that this derives at least in part from our fairly promiscuous evolutionary history. (To clarify, they use the term ‘promiscuous’ not in a judgmental way, but merely to convey having multiple sex partners). Their main premise is that rigid monogamy became common only after our ancestors made the shift from hunting and gathering to agriculture. With agriculture came an emphasis on fixed settlements, private property that could be inherited, genetic paternity, and female sexual fidelity. They argue that this stands in contrast to our hunting-gathering past, when sexual relationships were more open and not confined to an exclusive pair-bond.

Bonobo sex. From Lola ya Bonobo

As a corollary, they note that agriculture is a very recent event, adopted at different times in different places (or not at all) only within the last 10,000 years. For perspective, our species has been around for 200,000 years (McDougall et al, 2005), and our genus (Homo) for about 2.5 million years. The implication is that, although culture and environment certainly impact behavior, old behaviors may carry over into the present. They are quick to note that they do not negate the importance of romantic love in our prehistory. Nor are they arguing for biological determinism. They are merely saying that we have overlooked the fact that we have a long history as hypersexual animals, and that this is salient to modern relationships.

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Certain signposts in our biology and behavior point in this direction. These include, among other things: “our extravagant sexual capacity, ubiquitous adultery in allcultures, rampant promiscuity in both our closest primate relatives [chimpanzees and bonobos], (and) the absence of any monogamous primate living in large social groups” (Ryan and Jethá, p. 142). The purpose here is to explore some of these points, as well as others made in their book and by various researchers, that suggest a human proclivity for promiscuity. It is impossible to recap the entire literature on this in a single blogpost, but below are a few points…

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Traits suggesting promiscuity

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1. The absence of any monogamous primate living in social groups. First,Fuentes (1998) noted that there is a ‘semantic haze’ around ethological definitions of monogamy. Helpfully, Reichard (2003) proposed distinguishing between social monogamy (close physical proximity of a pair), sexual monogamy (exclusive mating), and genetic monogamy (sex occurs outside the primary pair-bond, but not reproduction). With this in mind, Fuentes stated that we won’t have a full understanding of how common sexual/ genetic monogamy is in primates until genetic testing becomes more common. He concluded that only about 3% of the 200-plus primate species were monogamous (pair-bonded adults living exclusively in two-adult groups), which is the same rate found in mammals generally. These seven species included two genera of New World monkeys (Callicebus, Aotus) and two lemur species.

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Humans, of course, are highly social and do not live in exclusive two-adult groups. To Fuentes, “there is strong evidence that the average group in humans is multi-male/multi-female and that group size remained at or below the 90-220 individual range until the upper Paleolithic” (p. 897). Ryan and Jethá have a point here: if monogamy is the default human mating behavior, then we need to account for why we are bucking such a strong primate (indeed, animal) trend.

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Furthermore, Reichard also pointed out that monogamy in other species varies in terms of duration, and lifelong mating appears to be exceedingly rare (one of the best examples may be in schistosomes).1 Even among monogamous gibbons, mating does not last for life, and in one study about 12% of copulations occurred outside the primary pair-bond (Fuentes, p. 896).

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2. Promiscuity in chimpanzees and bonobos. It is widely known that chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) are our closest relatives. However, from these species’ point of view, humans – not gorillas or orangutans – are their closest relatives too. Together, the three species lie on a branch on the evolutionary family tree referred to as the tribe Hominini (see Wood 2010 for a review). Chimpanzees and bonobos both live in multi-male/multi-female groups and are highly promiscuous, though there are substantive differences in sexual behavior between the two species. Chimpanzees generally confine sex to periods when females are in estrus. By contrast, bonobos have sex at any point of the ovulation cycle, invarious positions and partner combinations(including same sex), and as young as infancy (Woods and Hare 2011).

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This is relevant, since “only two species can do it week in and week out for nonreproductive reasons: one human, the other very humanlike (bonobos)” (Ryan and Jethá, p. 85). That’s probably not a coincidence. For bonobos, sex is frequent and has a social function, particularly in relieving tensions and as a bonding mechanism. In fact, Sue Savage-Rumbaugh once said that bonobos are rarely found in zoos because their sexual behavior is too human-like. Maybe. As Ryan and Jethá suggest, it is tantalizing to think that since promiscuity is found in both of our closest cousin species, and seems to be the case for humans as well, that it could have been the way our last common ancestor behaved.

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Primate family tree. The most recent common ancestor of humans, chimps, bonobos dates to about 6 million y.a., circled in red (click to enlarge).

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3. Our extravagant sexual capacity. Ryan and Jethá highlight the rather basic observation that human sexual activity goes well beyond what is needed for reproduction, with up to thousands of copulations per child born, and surpasses that of most other primates. Eric M. Johnsonput it this way: “there’s little denying that the human animal is one sexy beast.” They propose that, as in bonobos, sex in humans took on its current form not only in the pursuit of reproduction or physical pleasure confined to a pair-bond, but as an important means of social bonding within a tightly knit band of hunter-gatherers.

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My take is that a few milestones in evolution make this scenario plausible. (1) The origins of sex are unquestionably rooted in reproduction, likely as a means of creating variation in offspring. (2) At some point sex became pleasurable for animals, providing an extra incentive to pursue it, with fertilization then becoming a byproduct. An analogy I once heard – I forget where – is that this is akin to a farmer dangling a carrot (pleasure) in front of a mule to get it to work (reproduction). (3) For humans and bonobos, reproduction and sexual pleasure became sufficiently decoupled to the point where pleasure/ bonding became the goal and reproduction often took a back seat (mostly for the carrot, not the work). If that isn’t obvious, consider the many forms of sex where procreation is impossible (you’ll have to use your imagination here; my mom might read this). In fact, humans often go well out of their way to prevent sex from resulting in pregnancy.

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There might even be a 4th milestone, where sex became even farther removed from reproduction and entered some level of semi-rational conscious choice. Research suggests that people have sex for a variety of reasons other than reproduction. Apparently, there are 237 of them, roughly falling into four categories and thirteen subcategories (Meston and Buss, 2007). These include: Physical (stress reduction, pleasure, desirability, experience seeking), Goal Attainment (resources, social status, revenge, utilitarian), Emotional (love/ commitment and expression), and Insecurity (self-esteem boost, duty/pressure, mate guarding). Who knew?

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(As a side note, this post probably comes across as heteronormative, but I think findings such as those above, showing the many reasons people have sex other than for reproduction are obviously equally applicable to homosexual behavior. This is yet another reason that socially conservative arguments against homosexuality are hollow).

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4. Ubiquitous adultery in all cultures. Whatever we call it – adultery, infidelity, cheating – people have sex outside of marriage, and not at a trivial rate. Quoting Buss and Shackelford (1997: 194) “empirical estimates of affairs over the course of a marriage range from 30 to 60% for men and from 20 to 50% for women… Estimates of inﬁdelity over the course of a single year … yield lower estimates such as 5%.” I’m skeptical of the higher estimates, but even if the lower end is accurate these rates are substantial (and who knows how accurate they are since people are unlikely to reveal such intensely private information, even on anonymous surveys).

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What do we make of this? Buss and Shackelford found that traits correlating with infidelity include sexual dissatisfaction in the marriage, but also narcissism and low conscientiousness. Ryan and Jethá, however, feel that sexual novelty itself is a major reason people stray, in keeping with an ancestral pattern of promiscuity (p. 295). In some cultures this is permissible, or even encouraged and ritualized, while in others it is forbidden.

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5. The Ethnographic Record. As commonly mentioned in most introductory cultural anthropology classes, monogamy (or serial monogamy) is the most common form of marriage for individuals in the world, but most cultures permit some degree of polygamy. Murdock’s (1967) Ethnographic Atlas categorized just 16% of 862 cultures as exclusively monogamous, with polygamy being found at some level in the rest. Anupdated versionof the Atlas found a similar rate of about 15% (see Table 9 in the link). Culturally sanctioned marriage obviously does not represent the full range of sexual and mating behavior in any society (you probably knew that already), but it is shorthand for what practices are permissible.

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Looking for patterns among different types of human societies (i.e., hunter-gatherers, horticulturalists, pastoralists, agriculturalists), Marlowe (2000) noted that hunter-gatherer societies almost always allowed polygyny, and that “all else equal, the greater the level of paternal investment (in offspring), the more monogamous the mating system” (p.58). He also found that female extra-marital sexual relationships were fairly common in all types of societies except for agriculturalists, and that female affairs were negatively correlated with levels of male aggression in a given society.

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This helps corroborate Ryan and Jetha’s argument that concern with paternity and female sexual fidelity accompanied agriculture. They go further into the ethnographic literature, looking at populations like the matrilineal Mosuo in southern China, where adults of both sexes retain sexual autonomy throughout adulthood, and jealousy is culturally frowned upon. Other societies include the ‘partible paternity’ Native cultures in South America, where it is believed that many men can simultaneously be the father of a single child. The ethnographic literature is too vast to do it any justice here. However, culture truly is a powerful force.

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6. Multiple partners across cultures. At best, the closest humans come to monogamy would probably be serial monogamy. A 2011 study from the U.S. Centers for Disease Control found that just 23% of women and 14.7% of men aged 25-44 had one (or zero) opposite-sex lifetime partners (Chandra et al 2011). At the other end of the distribution, 30.5% of women and 49.4% of men reported having seven or more lifetime partners. A British study found a roughly comparable pattern (Johnson et al 2001). Similarly, the Kinsey Institute looked at 2,018 heterosexual, partnered adults (median age 50+) from the U.S., Germany, Japan, Brazil and Spain, and found that the mean number of lifetime sexual partners was 11.9 for men and 4.9 for women (Heiman et al 2011). However, there was lots of variation around the average for both sexes, ranging from 1 to 83 partners. Surveys from Zimbabwe, Uganda and South Africa found, consistently, that men aged 35+ years had an average of 6 or more lifetime sexual partners, while women averaged 1.3 to 3.6 partners (Todd et al 2009).

The above studies have a few limitations: they did not specify whether sex partners were concurrent or sequential, casual or committed, and while there is some cultural variation in the samples, it is obviously not an exhaustive list. Nonetheless, exclusive lifetime mating does not seem to be the norm.

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Some have wondered whether the large gender gap between the number of reported partners is legitimate, or whether men and women adjust their survey answers to conform to cultural expectations (Wiederman 1997). According to classic parental investment theory, female mammals should be more discriminating in the number of partners because they have more to invest in pregnancy, gestation, and care of offspring (Trivers 1972). However, this is not the case at all times in all species. In fact, while some have argued that serial monogamy benefits male reproduction as a form of sequential polygyny, Monique Borgerhoff Mulder (2009) found that in the Pimbwe, a horticultural population in Tanzania, females were the main beneficiaries from serial monogamy in terms of the number of surviving children.

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Furthermore, parental investment theory implies that sex and reproduction are synonymous, but in humans the two obviously do not overlap completely, as seen above. In one study of mostly white American college students (admittedly, a limited sample), the mean number of desired sex partners over the next year, given no health or social risks, or opportunity limitations, was 12.9 for men and 4.9 for women (Fenigstein and Preston, 2007). When all risks were taken into account, the means were much closer, but men still desired more partners (2.4 vs. 1.5). However, the data also showed that a substantial percentage of both sexes desired more than one partner in the coming year (47% of women; 76% of men). Why? Who knows. It could be any number of 237 reasons.

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We’re scratching the surface here, but this post is already too long. Overall, I think Ryan and Jethá and others put together a strong case that promiscuity is built into us to some extent, though this is not the entire story. More to come…

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Footnote:

1.) In her wonderfully quirky book, Olivia Judson lists other examples of species that fit the definition of true genetic monogamy, including jackdaws, chinstrap penguins, Kirk’s dik-dik, the California mouse, and some termites (2002: 153).

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13 thoughts on “Part 2. Humans are (Blank) -ogamous: Promiscuity”

Chimpanzees generally confine sex to periods when females are in estrus.

My understanding is that this is true for bonobos as well if we are talking about completed intercourse (see e.g. Craig Stanford’s “The Social Behavior of Chimpanzees and Bonobos”). Most bonobo sex outside of estrus does not involve intromission and/or is not continued to completion. Actually, most bonobo sex is between two females.

This is relevant, since “only two species can do it week in and week out for nonreproductive reasons: one human, the other very humanlike (bonobos)”

All simians appear capable of having sex outside periods of estrus. The ability to copulate on a situation-dependent rather than just a cyclical basis is part of evidence for Hrdy’s “paternity uncertainty” theory.

Buss and Shackelford found that traits correlating with infidelity include sexual dissatisfaction in the marriage, but also narcissism and low conscientiousness. Ryan and Jethá, however, feel that sexual novelty itself is a major reason people stray, in keeping with an ancestral pattern of promiscuity

Perhaps people who are narcissistic and less conscientious also happen to be higher in novelty-seeking.

Marlowe’s paper corroborates the idea that _society’s_ concern with paternity accompanied agriculture. Ryan seems to be arguing that men’s personal concern with paternity also accompanied agriculture, which requires a kind of human exceptionalism. All the other great apes demonstrate jealousy, male-male contests, mate guarding, etc., to some extent even if they contribute nothing more to their offspring than a few squirts of DNA.

Also, while the information on industrial societies is speculative, it doesn’t support the theory that concern with paternity is linked to private property. Based on the table, it seems like agriculturalists may be an anomaly (perhaps women are easier to sequester in agricultural societies?) or there may be a parabolic relationship between wealth variation and female affairs (foragers have the second lowest rate).

if monogamy is the default human mating behavior, then we need to account for why we are bucking such a strong primate (indeed, animal) trend

Maybe raising highly dependent offspring in a complex society?

reproduction and sexual pleasure became sufficiently decoupled to the point where pleasure/ bonding became the goal

I’m curious why you link sex and bonding here. The only theories with which I’m familiar suggest something mediated by oxytocin, which you discuss in the part on Pair Bonding and which leads me to believe that the idea that sex creates strong bonds between people derives as much from culture as biology.

the matrilineal Mosuo in southern China, where adults of both sexes retain sexual autonomy throughout adulthood, and jealousy is culturally frowned upon

Promiscuity also appears to be culturally frowned upon. From Judith Stacey’s Unhitched: “If Mosuo culture discourages displays of jealousy and anger, it does not seem to promote promiscuity or to honor fickleness. Small-scale societies exert potent indirect influences on behavior through gossip and reputation. The docent at the Mosuo cultural museum claimed that individuals earn bad reputations if their romantic behavior appears too licentious or selfish and that companionship, loyalty, and affection cement walking marriages as much as eros does.”

On sex and bonding, I didn’t mean that they went together automatically in every instance, nor that these bonds are necessarily long-term. The main point was that reproduction doesn’t seem to be the main objective in human sexuality. To clarify things, I could have made a minor grammatical change and rewritten this sentence as “reproduction and sexual pleasure became sufficiently decoupled to the point where pleasure AND/OR bonding became the goal.”

However, while culture is enormously important in how sex is perceived, I do think that for such an intimate activity between two sentient beings that it is likely there is some bonding involved for most people, even if it is fleeting. Again, I know that needn’t be true. If one uses ‘bonding’ in the most general sense, as the emotional reward gained from a shared experience, then it probably is applicable. Though I admit that may be semantics. Writing about evolution of human sexuality, the anthropologist Greg Laden had this to say:

“Yes, folks, compared to Pan trogoldytes, our nearest relative, human male sex is all about relationships. If you were thinking otherwise, this would be a good time to recalibrate.”

He has an interesting explanation for that, which I aim to incorporate into the conclusion.

Lastly, I think we may be talking past each other with the term ‘promiscuous.’ As mentioned here in Part #2, I’ve been using it in the way Ryan and Jetha did, which is merely to convey -without judgment- having more than one sexual relationship simultaneously (or near simultaneously). I didn’t mean to imply those relationships were indiscriminate or infinite in number. Perhaps I need a better word, since the common use is so loaded. I imagine that most cultures have a line between openness and licentiousness, but I don’t know how high that number goes in a given culture (or among individuals) before it crosses some acceptable boundary.

On sex and bonding, I didn’t mean that they went together automatically in every instance, nor that these bonds are necessarily long-term.

Right, which means that bonding is rather incidental to sex. Basically, I’m trying to understand what aspect(s) of sex you’re implicating as “important means of social bonding within a tightly knit band of hunter-gatherers.”

I imagine that most cultures have a line between openness and licentiousness, but I don’t know how high that number goes in a given culture (or among individuals) before it crosses some acceptable boundary.

Agreed, but that line is not necessarily the best indicator of sexual norms in a culture. In the case of the Mosuo, there is evidence that they are tolerant of promiscuity. However, there is little support for the depiction of them as characterized by simultaneous and near-simultaneous sexual relationships.

Perhaps I need a better word, since the common use is so loaded.

It’s unfortunate that the quote I used earlier contained the word promiscuous as my point seems to have gotten lost in semantics. The negative connotations of the common usage barely register with me. Since we’re on the subject, I favor any definition that doesn’t significantly deviate from those applied to other promiscuous species. To that end, I might quibble with the word “relationship” in your definition as having too strong an emotional resonance.

I’m more concerned with the lack of a definition for monogamy. The common definition of monogamy also distorts (not to mention politicizes) the concept. Monogamy needs to be as value-neutral as promiscuity. As Greg Laden notes in that link, “I find that discussions of primate sexuality often break down the moment someone not versed in the subject learns that monogamy is often not as monogamous as they thought. The fact that monogamy is not what you thought does not mean that it does not exist.”

Slight (but only slight) aside – as I recall, while chimps only mate during estrus, they have an extended estrus cycle so as to hide their ovulation. So they are having sex both to mate AND to protect offspring and increase social bonds. So when you discuss biological mating patterns, you should consider them as well.

“Basically, I’m trying to understand what aspect(s) of sex you’re implicating as “important means of social bonding within a tightly knit band of hunter-gatherers.”

Any form of social group requires cooperative efforts between members. A sexual social bond could serve to reinforce a sense of connection separate from the technicality of reproduction or short-term pleasure. It would emphasize a shared experience beyond raising children–emphasizing instead working toward a common goal: survival.

These conversations would go a lot more smoothly if we banned the use of “monogamy” in its unmodified form. Specifying whether you mean social or sexual monogamy is a big help. For instance, the (socially) monogamous prairie vole of oxytocin-bonding fame is often happy to take advantage of an opportunity for extrapair sex. As Tom Insel said, “they’ll sleep with anyone, but they’ll only sit by their partners.”

That said, I don’t agree that bonding is “incidental to sex”. Sexual arousal elevates oxytocin and AVP levels in people and activates the same brain regions involved in vole bonding. It would be helpful to have more human-specific information about these mechanisms, since both peptides have a variety of functions across even closely related species. But given the biology and the near-universal presence of romantic love — and its common association with sexual activity — cross-culturally, it seems unlikely that pair-bonding derives largely from culture. (Not that people do anything that isn’t influenced by culture to some degree.) Bonding doesn’t have to happen at every opportunity, nor last forever, to have adaptive significance.

I agree that bonding has adaptive significance, but I am suggesting that socialization, previous experiences, maybe even expectations can modify the mental and behavioral effects of the underlying biology in a manner consistent with the many and varied ways in which we use sex.

Basically, I’m trying to understand what aspect(s) of sex you’re implicating as “important means of social bonding within a tightly knit band of hunter-gatherers.”

Here I was referring to Ryan and Jetha’s argument that sex would have been a means of sharing pleasure and thus facilitating social relationships. They didn’t elaborate much beyond that. I suppose that pleasure could be strictly physical, but would likely have some emotional component as well. In their framing of things, the common hunter-gatherer ethic of ‘fierce egalitarianism’ applied not only to food and material wealth, but also to social relationships (and perhaps sexual ones) in a group where you would see the same people on a regular basis. I’m still not sure how I feel about this idea. Yes, sharing sex could act as a form of reciprocity through exchanging pleasure, but the ethnographic record also documents jealousy and some type of marriage in hunter-gatherers as well so sharing of wealth, partners would never be perfect. Smith et al (2010) also found that contemporary hunter-gatherer groups are not quite as fiercely egalitarian in wealth transfer as once thought (overall, they have Gini coefficients of inequality about at the same level as Denmark, which is about as egalitarian as a modern nation-state can be, but not, as they put it ‘primitive communism’). On the other hand, there are many reasons why it is problematic to use contemporary hunter-gatherers as models for our Pleistocene ancestors.

In the case of the Mosuo, there is evidence that they are tolerant of promiscuity. However, there is little support for the depiction of them as characterized by simultaneous and near-simultaneous sexual relationships.

That makes sense. Thanks for your insights on the Mosuo. Is there a reference you’d recommend?

I might quibble with the word “relationship” in your definition as having too strong an emotional resonance.I’m more concerned with the lack of a definition for monogamy. The common definition of monogamy also distorts (not to mention politicizes) the concept. Monogamy needs to be as value-neutral as promiscuity. As Greg Laden notes in that link, “I find that discussions of primate sexuality often break down the moment someone not versed in the subject learns that monogamy is often not as monogamous as they thought. The fact that monogamy is not what you thought does not mean that it does not exist.”

You and Sandra are right – definitions are stumbling blocks. I made a mental note of Greg Laden’s quote as well. I did mention Fuentes’ referring to the ‘semantic haze’ surrounding the term monogamy above, and the distinctions between social and sexual/genetic monogamy. However, in the rest of the post, I ignore my own warning however and drift back to a generic layman definition of monogamy. That is largely my own fault, but some stems from the fact that a lot of the literature is not very specific. It is kind of frustrating. Someone (Dan Savage?) referred to people being ‘monogamish,’ which is somewhat useful term in that it implies that there is a tendency to be sexually/socially monogamous, while also leaving some maneuverability and avoiding an overly strict definition. If gibbons (or voles or swans, or whatever) occasionally have sex outside of the primary relationship, does that automatically expel them from being called ‘monogamous’? I’m just being rhetorical here, but the same principle could apply to humans. Perhaps the definitions themselves are part of the problem, because they prejudice us to expect complex biological organisms to adhere to someone’s idealized version of sexual/ social behavior.

Thanks for your insights on the Mosuo. Is there a reference you’d recommend?

Tami Blumenfield has a fact sheet (PDF) about Mosuo marriage. The chapter on the Mosuo in Unhitched by Judith Stacey gives an interesting and informative overview; besides recounting her own experience, the chapter relies largely on the work of Chuan-kang Shih and Eileen Walsh.

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